In electronic-component-mounting apparatuses, a line sensor is used as image reading means for recognizing a position of an object to be mounted by image processing. A line sensor is formed of pixels arrayed in a line and the pixels are equipped with photoelectric transfer elements storing electric charges responsive to an amount of received light. When an optical image of an object is formed on the line sensor with an optical system, electric charges are stored in each pixel corresponding to the optical image of the object. Electric charges are output as an electric signal sequentially, so that one dimensional image data in pixel array direction, namely main scanning direction, can be obtained. Components are moved relative to sub scanning direction crossing main scanning direction, so that a plurality of one-dimensional data are arranged in parallel, whereby desirable two-dimensional image data is obtained. Conventionally a charge coupled device (CCD) line sensor has been used as this line sensor discussed above.
Components to be mounted have various kinds and sizes. A multifunctional-component-mounting apparatus for various components ranging from a small size to a large size is required to read images of components in various sizes with one image reading device. For this reason, an image reading device with a line sensor is usually equipped with a line sensor of which scanning width accommodates to the largest component. An image signal from the line sensor is stored in an image memory of the image reading device.
However, when an image is read, a CCD line sensor used in a conventional image reading device is required that electric charges be transferred from every pixel of the line sensor regardless of a component size. Such a conventional method takes excess time to read image because electric charges are transferred from pixels carrying no necessary information. A relative moving speed in sub scanning direction is sometimes dispersed by a mechanical error, and resolution in sub scanning direction is also dispersed resulting from this dispersion. An exact image thus can't be obtained.